Editing Triboelectric effect (section)

=== Liquids and gases ===
[[File:Tribocharge generated from a sliding drop.tif|thumb|Illustration of tribocharge generated from a sliding drop]]
The generation of static electricity from the relative motion of liquids or gases is well established, with one of the first analyses in 1886 by [[Lord Kelvin]] in his [[Kelvin water dropper|water dropper]] which used falling drops to create an electric generator.<ref>{{Cite journal |last=Thomson |first=W. |date=1868 |title=XVI. On a self-acting apparatus for multiplying and maintaining electric charges, with applications to illustrate the voltaic theory |journal=Proceedings of the Royal Society of London |volume=16 |pages=67–72 |doi=10.1098/rspl.1867.0019 |s2cid=110760051 |issn=0370-1662|doi-access=free }}</ref> Liquid mercury is a special case as it typically acts as a simple metal, so has been used as a reference electrode.<ref name=":34">{{Cite journal |last=Freund |first=Thomas |date=1979 |title=Tribo-electricity |url= https://linkinghub.elsevier.com/retrieve/pii/0001868679800032 |journal=Advances in Colloid and Interface Science |volume=11 |issue=1 |pages=43–66 |doi=10.1016/0001-8686(79)80003-2}}</ref> More common is water, and electricity due to water droplets hitting surfaces has been documented since the discovery by [[Philipp Lenard]] in 1892 of the ''spray electrification'' or ''waterfall effect''.<ref>{{Cite journal |last=Lenard |first=Philipp |date=1892 |title=Ueber die Electricität der Wasserfälle |journal=Annalen der Physik und Chemie |volume=282 |issue=8 |pages=584–636 |doi=10.1002/andp.18922820805 |bibcode=1892AnP...282..584L |issn=0003-3804|url=https://zenodo.org/record/1587780 }}</ref><ref>{{Cite book |last=Loeb |first=Leonard B. |url= http://link.springer.com/10.1007/978-3-642-88243-2 |title=Static Electrification |date=1958 |publisher=Springer |isbn=978-3-642-88245-6 |location=Berlin / Heidelberg |doi=10.1007/978-3-642-88243-2}}</ref> This is when falling water generates static electricity either by collisions between water drops or with the ground, leading to the finer mist in updrafts being mainly negatively charged, with positive near the lower surface. It can also occur for sliding drops.<ref>{{Cite journal |last1=Helseth |first1=L. E. |last2=Wen |first2=H Z |date=2017 |title=Visualisation of charge dynamics when water droplets move off a hydrophobic surface |url= https://iopscience.iop.org/article/10.1088/1361-6404/aa82f7 |journal=European Journal of Physics |volume=38 |issue=5 |page=055804 |doi=10.1088/1361-6404/aa82f7 |bibcode=2017EJPh...38e5804H |s2cid=125757544 |issn=0143-0807}}</ref>

Another type of charge can be produced during rapid solidification of water containing ions, which is called the ''Workman–Reynolds effect''.<ref>{{Cite journal |last=Gross |first=Gerardo Wolfgang |date=1965 |title=The Workman–Reynolds effect and ionic transfer processes at the ice-solution interface |journal=Journal of Geophysical Research |volume=70 |issue=10 |pages=2291–2300 |doi=10.1029/jz070i010p02291 |bibcode=1965JGR....70.2291G |issn=0148-0227}}</ref> During the solidification the positive and negative ions may not be equally distributed between the liquid and solid.<ref>{{Cite journal |last=Aziz |first=M. J. |date=1982 |title=Model for solute redistribution during rapid solidification |url= https://pubs.aip.org/jap/article/53/2/1158/11000/Model-for-solute-redistribution-during-rapid |journal=Journal of Applied Physics |volume=53 |issue=2 |pages=1158–1168 |doi=10.1063/1.329867 |bibcode=1982JAP....53.1158A |issn=0021-8979}}</ref> For instance, in thunderstorms this can contribute (together with the waterfall effect) to separation of positive hydrogen ions and negative hydroxide ions, leading to static charge and [[Lightning#Electrification|lightning]].<ref>{{Cite journal |last=Illingworth |first=A. J. |date=1985 |title=Charge separation in thunderstorms: Small scale processes |url=http://doi.wiley.com/10.1029/JD090iD04p06026 |journal=Journal of Geophysical Research |language=en |volume=90 |issue=D4 |page=6026 |doi=10.1029/JD090iD04p06026 |bibcode=1985JGR....90.6026I |issn=0148-0227}}</ref>

A third class is associated with contact potential differences between liquids or gases and other materials, similar to the work function differences for solids. It has been suggested that a triboelectric series for liquids is useful.<ref>{{Cite journal |last1=Yoo |first1=Donghyeon |last2=Jang |first2=Sunmin |last3=Cho |first3=Sumin |last4=Choi |first4=Dongwhi |last5=Kim |first5=Dong Sung |date=2023 |title=A Liquid Triboelectric Series |journal=Advanced Materials |volume=35 |issue=26 |pages=e2300699 |doi=10.1002/adma.202300699 |pmid=36947827 |bibcode=2023AdM....3500699Y |s2cid=257695984 |issn=0935-9648|doi-access=free }}</ref> One difference from solids is that often liquids have charged [[Double layer (surface science)|double layers]], and most of the work to date supports that ion transfer (rather than electron) dominates for liquids<ref>{{Cite journal |last1=Wong |first1=William S. Y. |last2=Bista |first2=Pravash |last3=Li |first3=Xiaomei |last4=Veith |first4=Lothar |last5=Sharifi-Aghili |first5=Azadeh |last6=Weber |first6=Stefan A. L. |last7=Butt |first7=Hans-Jürgen |date=2022 |title=Tuning the Charge of Sliding Water Drops |journal=Langmuir |volume=38 |issue=19 |pages=6224–6230 |doi=10.1021/acs.langmuir.2c00941 |issn=0743-7463 |pmc=9118544 |pmid=35500291}}</ref> as first suggested by [[Irving Langmuir]] in 1938.<ref>{{Cite journal |last=Langmuir |first=Irving |date=1938 |title=Surface Electrification Due to the Recession of Aqueous Solutions from Hydrophobic Surfaces |url= https://pubs.acs.org/doi/abs/10.1021/ja01272a054 |journal=Journal of the American Chemical Society |volume=60 |issue=5 |pages=1190–1194 |doi=10.1021/ja01272a054 |issn=0002-7863}}</ref>

Finally, with liquids there can be flow-rate gradients at interfaces, and also viscosity gradients. These can produce electric fields and also polarization of the liquid, a field called [[electrohydrodynamics]].<ref>{{Cite journal |last=Papageorgiou |first=Demetrios T. |date=2019 |title=Film Flows in the Presence of Electric Fields |journal=Annual Review of Fluid Mechanics |volume=51 |issue=1 |pages=155–187 |doi=10.1146/annurev-fluid-122316-044531 |bibcode=2019AnRFM..51..155P |s2cid=125898175 |issn=0066-4189|doi-access=free }}</ref> These are analogous to the electromechanical terms for solids where electric fields can occur due to elastic strains as described [[#Electromechanical contributions|earlier]].